1. Field of the Invention
The present invention relates to a method for determining an operational condition of a thermal field electron emitter used as an electron source for an electron microscope, an electron beam lithography system, an electron beam tester, a wafer inspection equipment, an Auger electron spectrograph or the like; a method for operating the thermal field electron emitter, and an electron beam utilizing system having the thermal field electron emitter.
2. Discussion of the Background
In recent years, in order to obtain electron beams of higher brightness, there has been used a thermal field electron emitter using a needle electrode of single crystal tungsten. This thermal field electron emitter is so constructed that a coating layer composed of zirconium and oxygen (hereinbelow, referred to as the ZrO coating layer) is formed on a single crystal tungsten needle having an axis direction of  less than 100 greater than  so that the work function of a (100) face of the single crystal tungsten is reduced from 4.5 eV to about 2.8 eV by the effect of the ZrO coating layer. The thermal field electron emitter has features of providing electron beams of high brightness, because only the fine crystal face corresponding to the (100) face formed on a tip of needle constitutes an electron emission region, and a long life in comparison with a conventional thermionic electron emitter. Further, it has features of being stable, operable in a bad vacuum condition and easy to use in comparison with a cold field electron emitter.
Although the proposed thermal field electron emitter has the above-mentioned advantages, it has such a drawback that a low frequency component of noise is large. An emission current contains a flicker noise (1/f noise) of about 0.23% in a range of 1 Hz-5000 Hz. However, it is known that the probe current contains a current variation of higher level than the flicker noise in a low frequency region and the low frequency component varies depending on temperature, electrostatic field strength and the shape of the cathode (see, Shinku, Vol. 29, No.1 (1986) 13-25).
Tip temperature is an important parameter in determining an actual operational condition of the thermal field electron emitter. Generally, a tip is heated by Joule heat by feeding a direct current to the tungsten filament which is welded with a tip. Therefore tip temperature is controlled by filament current. For example, there is description that the optimum working temperature of the above-mentioned ZrO-coated tungsten thermal field electron emitter is 1800 K (see J. Applied Physics., 46, 5 (1975) 2029-2050). In a generally used method according to such information, the filament current is determined so as to correspond to a tip temperature of 1800 K. However, even in a single thermal field electron emitter, the heat dissipation varies depending on an electrode structure mounted, and accordingly, there appears a change of a set value of filament current which is used in order to obtain a predetermined tip temperature. The tip temperature can be measured with an optical pyrometer. However, in some cases, an electron beam utilizing system having no window for observing a tip temperature is used. Such electron beam utilizing system may not be able to detect an actual temperature. Further, even when an adjustment of 1800K is made, there is no evidence as to whether or not the thermal field electron emitter is at an operational temperature capable of obtaining a stable electron emission with little current fluctuation, because a large variety of heat dissipation takes place depending on an electrode structure used. As means for confirming the stability of electron beams, there is a method for detecting probe current drift when electrons emit continuously for a long term. However, the method has such problems that it takes much time, e.g., several tens hours for confirmation and it is not always successful by only one time of adjustment. Here, the probe current means an axial current.
It is an object of the present invention to provide a method for determining, in a short time, an appropriate operational condition of a thermal field electron emitter so that it can emit electrons stably.
It is an object of the present invention to provide a method for operating the thermal field electron emitter.
Further, it is an object of the present invention to provide an electron beam utilizing system having the thermal field electron emitter, which can improve operating availability of the thermal field electron emitter.
In accordance with the present invention, there is provided a method for determining an operational condition of a thermal field electron emitter, which comprises measuring an S/N ratio of a probe current while a filament current is changed, and determining the filament current in which the S/N ratio is maximal.
Further, in accordance with the present invention, there is provided a method for operating a thermal field electron emitter characterized in that the thermal field electron emitter is operated under the condition that the S/N ratio of a probe current is measured while a filament current is changed, and the filament current in which the S/N ratio is maximal is maintained.
In the above-mentioned methods, the thermal field electron emitter comprises a needle of single crystal tungsten having an axis direction of  less than 100 greater than  on which a coating layer composed of zirconium and oxygen is formed.
In accordance with the present invention, there is provided an electron beam utilizing system for determining an operational condition of a thermal field electron emitter, which comprises a thermal field electron emitter, a control electrode, an extractor, an electrode for receiving a probe current of electron beams which pass through the extractor, and a spectrum analyzer for measuring an S/N ratio of the probe current wherein the S/N ratio is measured by the spectrum analyzer while a filament current is changed, to determine the filament current in which the S/N ratio is maximal.
In the above-mentioned system, the thermal field electron emitter comprises a needle of single crystal tungsten having an axis direction of  less than 100 greater than  on which a coating layer composed of zirconium and oxygen is formed.